Because of statutory requirements, components of a motor vehicle which are relevant to the emission of toxic substances must be monitored with on-board equipment during operation. For example, faults are intended to be discovered and displayed which permit the toxic substance emissions to rise over the permitted values by a factor of 1.5. The possible fault sources include, inter alia, the complete or partial loss of the conversion capability of a catalytic converter for toxic substances. It is known that the conversion capability for toxic substances is associated with the capability of storing oxygen. Accordingly, a conclusion can be drawn as to the conversion capability from the oxygen storage capability.
In this context, U.S. Pat. No. 5,267,472 discloses to set, under defined conditions, the oxygen quantity introduced into the catalytic converter into relationship with the oxygen quantity flowing out therefrom and to draw a conclusion as to the oxygen storage capability of the catalytic converter therefrom. For this purpose, the oxygen content of the exhaust gas is detected forward and rearward of the catalytic converter and multiplied by the air mass flow through the engine or by the exhaust-gas mass flow through the catalytic converter and then the products are integrated. The integral values are an index for the oxygen quantities which flow to the catalytic converter and which flow out therefrom. The difference of the integral values supplies the change of the degree of oxygen charge of the catalytic converter in the integration time frame. In the known method, it is ensured that, during the diagnosis, a complete charge of the catalytic converter with oxygen and a subsequent complete discharge thereof and vice versa takes place.
Complete charges and/or discharges of the catalytic converter cause reactions of the rearward exhaust-gas probe and toxic-substance peaks downstream of the catalytic converter. A reaction of the rearward probe shows that the catalytic converter is overloaded with oxygen or indicates an oxygen deficiency. Toxic substances rearward of the catalytic converter then also occur. For example, oxygen deficiency in the exhaust gas forward of the catalytic converter is associated with the occurrence of hydrocarbons and carbon monoxide. As long as the catalytic converter can still give off oxygen, these toxic substances are converted in the catalytic converter to water and carbon dioxide with the participation of oxygen. A catalytic converter emptied of oxygen cannot make this conversion. The above-mentioned toxic substances, which occur in a reaction of the rearward probe, are therefore released to the ambient.